Optical compensation film, or retardation film, is known as a device for improving viewing angle of a liquid crystal display device. The optical compensation film is produced by forming an alignment film on a long web of transparent film, and then forming an optically anisotropic liquid crystal layer by spreading and drying a liquid crystal on the alignment film, as disclosed for example in Japanese Laid-open Patent Application No. Hei 9-73081. Although the manufacturing processes for the optical compensation film are strictly supervised, it is hard to exterminate any defect of the produced film. The film defect may be caused by various factors, like mixture or adhesion of extraneous matters, and includes irregularity of molecular orientation, uneven thickness of the base transparent film and uneven coating of the liquid crystal layer. For this reason, it is necessary to locate the detective position on the optical compensation film during the manufacture.
Defect inspection of the optical compensation film on the manufacturing line may be called the on-line inspection, and there are many conventional methods for the on-line inspection.
For example, Japanese Laid-open Patent Application No. Hei 6-235624 suggests an inspection method, wherein inspection light beams are projected from a light source toward a target transparent film as being conveyed, to receive reflected light beams or transmitted light beams on a linear sensor. Based on the received light beams, fine unevenness of the film surface, extraneous matters or air bubbles in the film, or protuberances produced on an antireflective coat on the film surface can be automatically detected at a high speed.
Japanese Laid-open Patent Application No. Hei 8-54351 discloses a defect inspection method, wherein a high-luminous high-directional light beam is projected at an angle of 5° to 15° onto a surface of a transparent sheet as being conveyed continually, and light transmitted through the transparent sheet is captured by a camera, so as to detect the film defect based on data obtained by image-processing an output signal from the camera. This method enables detecting fine unevenness of the film thickness, i.e. unevenness of 0.1 μm to 5 μm deep and 0.1 μm to 10 μm wide.
Japanese Laid-open Patent Application No. Hei 6-148095 discloses an inspection method, wherein a light source and a camera are placed in opposition to each other across a film to inspect, and a first polarizing plate is placed in front of the light source, whereas a second polarizing plate is placed in front of the camera. The film defect is detected based on data obtained by image-processing an output signal from the camera. Since the polarizing direction of the first and second polarizing plates is displaced by an angle of 20° or so from their vertical direction, it becomes possible to detect an increase in light intensity, which results from a change in polarizing condition, and occurs where an extraneous matter exits in or on the film.
Japanese Laid-open Patent Application No. Hei 11-30591 discloses a method, wherein a light source, a camera and first and second polarizing plates are arranged in the same way as in the above-mentioned Japanese Laid-open Patent Application No. Hei 6-148095, but the displacement of the polarizing direction of the polarizing plates is set to be not more than ±20°. Thereby, vertically polarized components, which are generated by irregular orientation of film molecular or fine distortion of the film, are reduced, so texture signals are lowered and local changes in transmitted light amount through the film are reduced. Then, a change in polarizing condition that occurs at a defective position is made apparent as a dark area signal.
However, any of the prior defect inspection methods cannot detect such a super fine defect that is measured as a thickness difference of 1 nm to 100 nm in film or coating thickness. The present applicant tried to detect uneven coating of a low reflective optical use film and an optical compensation film, using the inspection methods as disclosed in the above-mentioned Japanese Laid-open Patent Application Nos. Hei 6-235624 and Hei 8-54351. But it was hard to detect the super fine unevenness as above. Also with the prior art disclosed in the above-mentioned Japanese Laid-open Patent Application No. Hei 6-148095, satisfying results could not be obtained. Particularly, a change in polarizing direction that is caused by irregular molecular orientation or uneven thickness could not be detected in the vertical direction to the film, because S/N ratio of an output signal of the camera, which is a ratio between an amplitude of a detection signal for the normal texture and that for the defect, was so low that it was hard to discriminate between the normal portion and the defective portion. The present applicant also tried to detect coating unevenness of the film with the method of the above-mentioned Japanese Laid-open Patent Application No. 11-030591, where the polarizing direction of the polarizing plates was displaced not more than ±20° from the vertical direction to the film. However, it was found that the S/N ratio was worsened except where the displacement of the polarizing direction was zero degree, and the coating unevenness could hardly be detected even with the displacement of zero degree.